This invention relates to vinyl carbosiloxane resins and to their use as encapsulants, particularly for LED devices.
High power light-emitting-diodes (LEDs) are gaining momentum as a replacement for incandescent and fluorescent lamps for retail use, architectural illumination, automotive use, and street lighting. Heat resistant polymers and/or polymer composites are used as encapsulant materials, and are known to maintain mechanical properties (modulus, elongation, toughness, adhesive strength) under thermal aging conditions. These are important for LED applications, but without good optical transparency under continuous usage, the polymers are unsuitable.
Traditionally, epoxies have been the preferred encapsulants for LED encapsulation because they have low moisture permeability, high refractive index, high hardness, and low thermal expansion. Nevertheless, epoxies turn yellow after exposure to photon fluxes and temperatures at about 100° C. Due to high electricity consumption, LEDs can reach operating temperatures as high as 150° C.; consequently, light output from LEDs is significantly affected when epoxies are used.
Silicone based materials are known to withstand high temperature and photon bombardment without developing yellow coloration. However, for high brightness LED encapsulation, most commercially available silicones cannot provide the hardness needed for this application. Hardness is needed both for protection of the LED semiconductors and for optimum transparency (a soft surface holds dust more than a hard surface and affects light quality). A good silicone candidate is 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane.
This cyclic tetravinyl tetrasiloxane compound provides sufficient Shore D or high Shore A hardness when cured with hydridosilicones having multiple Si—H groups, a low coefficient of thermal expansion, and good transparency, all of which are important properties for LED encapsulants. The disadvantage is that cyclic tetravinyl tetrasiloxane is volatile and can be lost from the reaction mix during final cure.